Dye lasers excited with flashlamp were first discovered by Sorokin and Lankard in 1967. These flashlamp excited dye lasers have found use in many applications. The dye, which is the laser medium, is dissolved in a solvent, most usually of organic nature. The laser medium, being a solution, makes the flashlamp excited dye laser a liquid laser. The dye solution is circulated through a laser pump cavity by means of a capillary dye cell, the axis of which in most instances coincides with the laser axis. The dye cell is activated or excited by a flashlamp which is in close proximity to it. The ends of the capillary dye cell are terminated with laser windows through which the laser beam can be extracted.
The dye solution, comprised of the laser dye or dyes and organic solvent and which may include other chemical additives, undergoes photochemical changes induced by the flashlamp light. The photochemical action may result in the destruction of dye molecules and generation of by-products that absorb at the lasing wavelength and that reduce the gain of the laser for subsequent excitation pulses. To minimize the contribution of these deleterious reactions, a large reservoir of dye solution can be used to minimize the proportion of degraded dye solution. However, the deleterious by-products accumulate and, in time, the overall dye solution will degrade as the laser is used.
To overcome this problem, many different types of dye circulation systems have been devised either to minimize the generation of deleterious by-products, or to remove the deleterious by-products by means of filtering systems.
An ideal approach to keep the dye solution from degrading under use is to identify a filter that selectively removes the contaminant that degrades the laser output. A generic concept of such a circulation system was disclosed as U.S. Pat. No. 4,364,015 by Drake et al. Although the patent describes the circulation system in a generic manner, the exact nature of the selective filter that removes degradation by-products is not described; nor has such a filter been discovered that can universally be used with all dye laser solutions. Mostovnikov describes a filter that appears to have the properties of a selective filter (V. A. Mostovnikov et al., xe2x80x9cRecovery of lasing properties of dye solutions after their photolysis, American Institute of Physics, Sov. J. Quantum Electron, Vol. 6, No. 9, September 1976). Attempts to duplicate his approach in commercial dye lasers that require repetitive operation of tens of thousands of pulses have been unsuccessful.
It is unlikely that a universal selective filter can be discovered because there are infinite combinations of dyes, solvents, and additives used in dye lasers. The filter described in U.S. Pat. No. 4,364,015 to remove dye solute is identified as a charcoal bed filter. Charcoal is effective in removing most dye solutes used in flashlamp excited dye lasers. Charcoal bed filters have also been shown to be selective in removing deleterious by-products generated in dye laser solutions.
Another complication that arises in finding filters that remove dye solute or degradation products is the rate of degradation of the dye solution. Certain dye solutions degrade slowly and the degradation by-products contributed by each excitation pulse is low. Dye solution life is long, and simple degradation compensation schemes, such as increasing the excitation pulse to compensate for loss in gain produced by the degradation products, can be used. In other cases, the solution volume irradiated by the excitation pulse is so full of degraded by-products that it is best to discard the irradiated volume than send the irradiated volume back to the reservoir where it can contaminate the solution in the reservoir. A dye circulation system that extracts the excited and degraded solution in a single shot is described in U.S. Pat. No. 4,977,571 to Furumoto et al.
If a rapidly degrading dye solution is used with a dye circulation system described in U.S. Pat. No. 4,364,015, the flow in the cleaning loop must be increased to keep up with the degradation. The system will work but the flow in the bypass cleaning loop will increase to be equal to, or greater than, that in the loop that contains the laser head. If the flow in the cleaning loop is large, the metering pump must add a considerable amount of dye concentrate to keep the dye concentration at the optimum level. It has been known for some time that in a situation where the dye solution flow through the cleaning loop is large and the dye solute added is large, or if concentrate is added continuously without replacing the filter, the solute removing filter will begin to load up with dye solute and not be able to remove all of the dye solute coming into the filter. However, it was noted that the filter, if it is a charcoal filter, has the property of removing degradation by-products that reduced the gain of the laser as well as dye solute, even if it passed dye solute. The above observations were also noted by Garden et al. and presented in U.S. Pat. No. 5,109,387. That patent describes the filter as being saturated with dye solute and the dye solution is regenerated by the filter.
Experiments with charcoal bed filters indicate that filters do not saturate with dye but continually absorb dye solute, though at a diminishing rate, and the dye solute concentration in solution does not come into equilibrium to maintain a constant concentration. The filter does not continually regenerate the dye solution and, in fact, experiments show that dye and additives must be added as the filter is used.
The present invention relates to a method of replenishing dye solution in a dye laser. Prior to operation of the dye laser, a solid state porous system with restricted geometries is loaded with solute to act as a repository of dye solute. Specifically, solution at a predetermined operating dye solute concentration flowing into the porous bed filter, PBF, repository remains at substantially the same concentration. With firing of the laser, the PBF serves the necessary function of filtering out undesirable by-products of the lasing process. However, since it was preloaded, only minimal amounts of dye solute are filtered out. Solute concentration in the dye solution, which may include dye solute concentration and additive solute concentration, is monitored, and solutes are replenished as required in response to changes in the monitored solute concentration. Preferably, only dye concentration is monitored, but both dye solute and additive solute, such as cyclooctatetraene (COT), can be added together.
Since the filtering action of the PBF is very temperature dependent, temperature of the dye solution is monitored and controlled, preferably within xc2x12.5xc2x0 C. With the solute metering and close temperature control, color output of the laser is very stable and solute concentration can be closely controlled to maximize color output at the wavelength selected by the optical color regulator often found on dye lasers.
Preferably, dye solute concentration is monitored by monitoring optical absorption of the solution. In a preferred embodiment, light is directed from a light source through a first filter and through dye solution to a detector in a first channel and from the same light source through a second filter to a detector in a second channel. The monitor filters have a passband at a characteristic absorption wavelength of the dye solution and, unlike a spectrophotometer, reject broadband fluorescence of the dye solution. The outputs to the two channels are compared to provide the indication of dye concentration. The sensitivity of the two channels may be adjusted so that they yield the same signal strength when the concentration of the solute in solution is the desired operating concentration. A difference signal between the two channels may be digitized and loaded into an electronic counter with a solute metering pump being driven by the counter.
The monitoring and replenishing steps need not be performed continuously but may be performed after some predetermined number of firings of the laser. The number of firings may be adjustable to match solute degradation rate.
The solute concentration balance between the filter and the solution is highly temperature sensitive. Preferably, a temperature sensor is located in the circulation loop to monitor the temperature, and a temperature controller is used to control a heater and a cooling device to control the dye solution temperature.
The number of laser pulses that can be elicited from the dye solutions can be notably increased by maintaining the solution at a designated or preselected pH value. This pH maintenance is accomplished by adding one or more pH buffer substances to the solution. The addition of such buffers to the dye solution provides enhanced lasing process life to these solutions.